Bioactive glass-ceramics are compositions that are capable of inducing specific biological activity. Specifically, bioactive glass-ceramics are widely used bone repair materials due to their unique properties, such as osteoconductivity, osteoinductivity and biodegradability.
In early 1970s, Larry L. Hench reported on the bone bonding ability observed for the glass-ceramic system based on SiO2—CaO—Na2O—P2O5 which was given the trade name Bioglass® (Hench L L, et al., Bonding mechanisms at the interface of ceramic prosthetic materials. J Biomed Mater Res., 5:117-41 (1971)). Today a wide range of bioactive glass compositions have been developed through different synthesis routes and found a variety of applications including soft tissue bonding (Hench L L. Bioceramics. J Am Ceram Soc., 81:1705-28 (1998); Wilson J, Nicolletti D. Bonding of soft tissues to bioglass. In: Yamamuro T, Hench L L, Wilson J, ed. Handbook of bioactive ceramics: bioactive glasses and glass ceramics. Boca Raton Fla.:CRC Press, 283-302 (1990); and Wilson J, et al. Toxicology and biocompatibility of bioglasses. J Biomed Mater Res., 15:805-17 (1981)), lung tissue engineering (Tan A, et al. The effect of 58S bioactive solgel-derived foams on the growth of murine lung epithelial cells. Key Eng Mater., 240-242:719-24 (2003); and Verrier S, et al. PDLLA/Bioglass composites for soft-tissue and hard-tissue engineering: an in vitro cell biology assessment. Biomaterials. 25:3013-21 (2004)), inducing angiogenesis (Day R M. Bioactive glass stimulates the secretion of angiogenic growth factors and angiogenesis in vitro. Tissue Eng. 11:768-77 (2005)), stimulating the gene expression (Xynos I D, et al. Gene-expression profiling of human osteoblasts following treatment with the ionic products of Bioglass 45S5 dissolution. J Biomed Mater Res. 55:151-7 (2001)), and growth factor production in osteoblasts (Xynos I D, et al. Ionic dissolution products of bioactive glass increase proliferation of human osteoblasts and induce insulin like growth factor II mRNA expression and protein synthesis. Biochem Biophys Res Commun., 276:461-5 (2000)), etc.
Bioactive glass-ceramics can be synthesized by melt or solgel methods. The solgel method is a chemical synthesis technique that involves the conversion of a sol, which is a suspension of very small, colloidal particles to a three dimensional interconnected network termed gel. Six steps are usually involved in the preparation of bioactive glass-ceramics by this method. The first step involves the mixing of precursors, which form a low viscosity sol whose viscosity steadily increases as the network interconnect develops. Before the completion of the network formation, the sol is cast into a mold where gelation occurs as the third step.
The resulting three dimensional gel networks are completely filled with pore liquid. Aging is the fourth step, which involves holding the gel in its pore liquid for several hours. In step five, the gel is dried during which the pore liquid and the physically adsorbed water are completely eliminated from the pores. This involves heating at controlled rates at temperatures of 120-180° C.
Chemical stabilization of the gel, often called “calcination” is the sixth step that is necessary to remove the residual components and unwanted byproducts associated with the hydrolysis and condensation reactions. This stabilization step confers to the environmental stability and bioactivity. This step usually is a thermal treatment in the range of 500−900° C., which desorbs surface silanols (Si—OH) and eliminates other residuals from the gel. Usually temperature of calcination for bioactive glass-ceramics is selected to be 600° C. and it is quoted that maximum bioactivity is obtained with minimum stabilization temperature (Xynos I D, et al., Gene-expression profiling of human osteoblasts following treatment with the ionic products of Bioglass 45S5 dissolution. J Biomed Mater Res., 55:151-7 (2001); Xynos I D, et al., Ionic dissolution products of bioactive glass increase proliferation of human osteoblasts and induce insulin like growth factor II mRNA expression and protein synthesis. Biochem Biophys Res Commun. 276:461-5 (2000); and Jones J R, et al., Optimising bioactive glass scaffolds for bone tissue engineering. Biomaterials, 27:964-73 (2006)). According to Hench et al., calcination also has its effect on increasing the strength and hardness of the gels and converts the network to a glass with network properties similar to the conventional melt derived glasses (Hench L L, Wilson J. An introduction to bioceramics. In: Hench L L, Wilson, ed. Advanced series in ceramics, USA:World Scientific Publishing Co, 1-24 (1993).
Since the stabilization of the glass-ceramics (especially sodium based compositions) by conventional heat treatment alter the glass-ceramic properties like particle size, density, surface area, etc., similar to melt derived materials, stabilization of solgel derived structure of these glass-ceramics by another means would be advantageous in this respect. Additionally, these properties are significant in the field of composites as well, where the bioactive glass-ceramics serve as reinforcement in low elastic modulus polymeric matrix. Moreover, it is well reported that the textural features like particle size distribution, specific surface area, porosity, solubility, etc. have a strong influence on bioactivity. This is for the reason that, during the bone bonding mechanism the rate of formation of hydroxyl carbonate apatite (HCA) layer, the interfacial layer that is structurally and chemically equivalent to the mineral phase of the bone, is influenced with the particle size range and powder volume fraction.
Other approaches to stabilization of bioactive glass and to removing impurities have been studied and use ethanol washing (Mukundan L., et al. A new synthesis route to high surface area solgel bioactive glass through alcohol washing. 3(2) (2013))
A new approach to the processing of solgel derived glass-ceramics wherein the glass-ceramics are not subjected to the high temperature chemical stabilization process or solvent washing is described. Hence the current work presents an alternative method of preparing, stabilizing and maintaining the pore structure of the solgel-derived bioactive glass-ceramics.